The present invention relates to a male snap part and a female snap part constituting a snap fastener or a pair of snap buttons.
A snap fastener has been widely used for clothes, bags and the like, in which a cylindrical projection of the male snap part (male snap) is engaged with and disengaged from a projection receiving, cylindrical depression of the female snap part (female snap). Generally, the male snap's projection includes, at its distal end, an annular, outer bulge (male bulge) which bulges outward in the radial direction, while a cylindrical portion defining the female snap's depression includes, at its open end, an annular, inner bulge (female bulge) which bulges inward in the radial direction. In an initial state (a non-deformed state) of the male and female snaps, the outer diameter of the male bulge is slightly larger than the inner diameter of the female bulge. The projection of the male snap and/or the depression of the female snap have the capability to elastically deform (or flexibility) in the radial direction. When the male snap is attached to and detached from the female snap, the male and female bulges are engaged with each other, which deforms the former radially-inwardly and/or the latter radially-outwardly, making the outer diameter of the male bulge temporarily equal to the inner diameter of the female bulge. Once the male bulge has passed over the female bulge, they return to their respective initial states (the male and/or female bulges do not always completely return to the initial states when both snaps are coupled to each other).
Metallic male and female snaps are less deformable elastically than resinou snaps. To compensate for a shortfall of the elasticity, an additional element such as a C-shaped spring and a M-shaped spring has been added into a metal female snap, as disclosed in Japanese Examined Utility Model Application Publication Nos. H02-21929 and H03-54566, etc. However, with such an additional element, manufacturing steps and costs increase.
In metallic snaps, it has been known to provide a plurality of slits, in the circumference direction, into the projection of the male snap and/or the depression-defining portion of the female snap in order to make them more deformable elastically, as shown in U.S. Pat. Nos. 1,732,837 and 1,896,044, etc.
With the above-mentioned slits, however, it is difficult to obtain an appropriate elasticity. For example, if slits are formed in the male snap's projection relatively deeply from its top to the bottom, then the projection's elasticity will increase while its coupling strength (or detaching resistance) with respect to the female snap's depression decreases, causing the snaps to be detached too easily. On the other hand, if slits are short or shallow in the projection from its top, its elasticity may be insufficient, and therefore the force required in attaching and/or detaching the snaps rises too much.    [Patent Document 1] Japanese Examined Utility Model Application No. 02-21929    [Patent Document 2] Japanese Examined Utility Model Application No. 03-54566    [Patent Document 3] U.S. Pat. No. 1,732,837    [Patent Document 4] U.S. Pat. No. 1,896,044